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on-site renewables in Appendix G

asked 2016-11-09 10:17:43 -0600

Jim Dirkes gravatar image

updated 2017-04-02 20:06:46 -0600

Dear UH forum, I've not modeled a PV system that required compliance with 90.1 Appendix G (it's actually a 179d tax credit model), so I'm carefully reviewing the parts of 90.1 that I'm less familiar with. One of those parts is G2.4 (copied below).

In short, I do not understand what it's saying and how it impacts my preparation of the energy model.

  • I know how to include energy costs / tariffs of various types
  • I think it's saying that a cost credit should not be given for site-generated renewable energy (the owner just pays less to the utility)
  • I don't understand why (as it seems to say) one cannot include energy from renewables in the Proposed model
  • I don't know what is meant by a "backup energy source" and how that affects the Baseline model

All assistance appreciated. Thanks in advance.

G2.4 Energy Rates. Annual energy costs shall be determined using either actual rates for purchased energy or state average energy prices published by DOE’s Energy Information Administration (EIA) for commercial building customers, but rates from different sources may not be mixed in the same project.

Exception: On-site renewable energy sources or site-recovered energy shall not be considered to be purchased energy and shall not be included in the proposed building performance. Where on-site renewable or site-recovered sources are used, the baseline building performance shall be based on the energy source used as the backup energy source or on the use of electricity if no backup energy source has been specified.

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answered 2016-11-09 11:53:53 -0600

updated 2016-11-09 11:54:50 -0600

Perhaps these paragraphs from pages G-7 and G-8 of the (2007) Users' Manual clarifies:

There is a special case for calculating the energy cost for proposed buildings that have on-site renewable energy sources or site-recovered energy. For example, a building may have a solar thermal array, photovoltaic panels, or access to a geothermal energy source. Or a building with substantial refrigeration loads may recover heat from the condenser to meet service water heating loads. If the proposed building has either renewable or recovered energy, this energy is considered free energy by the building performance rating method, and that energy is not included in the proposed building performance rating.

The baseline building does not have on-site renewable energy source or heat recovery, except as required by G3.1.2.10 or Table G3.1-11b. For the baseline building, the loads which are met by renewable or recovered energy in the proposed building are considered to be served by the backup energy source.

"Backup energy source" just seems to be the non-renewable (e.g. grid-supplied) energy.

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I'm still missing something...

Are you saying or ASHRAE that if the building owner invests in PV, for example, there is no credit / savings for doing that? "that energy is not included in ... the rating" sounds like you cannot count the PV output in any fashion to get a better rating.

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-09 13:19:46 -0600 )edit

I think what it means is that renewable/recovered energy is 'free', and not included in the calculation of energy costs. I.E. you calculate the proposed energy cost by (Total energy consumed - renewable energy produced). So your proposed building PV system helps PRM performance, like we've all done successfully for LEED projects.

Eric Ringold gravatar imageEric Ringold ( 2016-11-09 13:30:11 -0600 )edit

My paraphrasing of G2.4 would be something like "Use average or real utility rates to calculate how much the proposed modeled energy costs. Except if some energy comes from renewable/recovered sources, then don't include that in the costs. Baseline costs use the same rate, and only discount renewable/recovered if it's required."

Eric Ringold gravatar imageEric Ringold ( 2016-11-09 13:41:35 -0600 )edit

Thanks, Eric. That makes perfect sense to me. It would never have occurred to me to do other than just use the renewables-reduced utility bill - which is what you are saying. I'm one of those who reads everything (eventually) multiple times to make sure I understand it, and that paragraph sure could be more clear...

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-09 14:03:16 -0600 )edit

I agree, and I find that about way too many sections in the standard.

Eric Ringold gravatar imageEric Ringold ( 2016-11-09 14:04:32 -0600 )edit
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answered 2016-11-09 14:10:14 -0600

updated 2016-11-09 14:23:29 -0600

The language of G2.4 in 90.1-2007 was not the best so we tried to clean it up some in 90.1-2013 (although reading the revised language it may not be the clearest either).

In my opinion, this is what paragraph means:

The energy costs for the proposed do NOT include any energy generated by PV. That makes sense, no energy cost for electricity generated by the PV. So if your building needs 100,000 kWh and the PV generates 30,000 kWh. You would only have the proposed building performance energy cost based on 70,000 kWh.

The baseline does not include PV in the model, so what energy source is used to satisfy the electric load for the building. Well the "backup" energy source for PV would be the electric utility. So this means that the entire baseline building performance energy cost is based on the building without PV. So using the same numbers as above, if the building needs 100,000 kWh, since there is no PV in the baseline model, the energy cost for the baseline should be based on 100,000 kWh.

Just like always, the difference between the baseline and proposed is the credit received.

For a non PV case, the "backup" concept may be more logical, for site recovered heat used to preheat water going into a gas boiler, if the site recovery was not available, more gas would be consumed so the "backup" source would be gas.

Please keep in mind that this is just my opinion and not anything official from ASHRAE.

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Hi. I have submitted one energy model for LEED Project in the last days that included PV and solar thermal system. I used the metodolgy commented by JasonGlazer, and the LEED reviewing was ok.

Vmoreno gravatar imageVmoreno ( 2016-11-09 14:41:35 -0600 )edit

That's very helpful. Thank you very much.

p.s., I take the opinion of someone on the 90.1 committee as a very informed opinion :)

Jim Dirkes gravatar imageJim Dirkes ( 2016-11-09 15:28:15 -0600 )edit

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Asked: 2016-11-09 10:17:43 -0600

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Last updated: Nov 09 '16